Production of 3,6-dichloropicolinic acid (3,6-D), a highly active plant growth regulator, by electrolytic reduction of tetrachloro-2-picolinic acid ("tet-acid") at a silver cathode in basic, aqueous solution is disclosed in U.S. Pat. No. 4,217,185 (the disclosure of which is hereby incorporated herein for all purposes sanctioned by United States patent laws).
The cathode has a surface layer of silver microcrystals formed by the electrolytic reduction of colloidal, hydrous, silver oxide particles in the presence of an aqueous base. The presence of other noble metals is not beneficial and the presence of base metals (nickel and copper, most notably) definitely lowers the activity of the cathode. However, other conductive metals, including stainless steel, may serve as a substrate on which the silver layer is formed, provided that the substrate metal not be subjected to anodization (as in a preferred method of forming the precursor silver oxide particles).
Inert electrode materials are generally suitable for the anode to be used in conjunction with the silver cathode, but in order to attain 3,6-D yields of 90% or more, it is necessary, according to the patent, to employ anodes consisting essentially of graphite. Other anode materials are believed to encourage decarboxylation ("Kolbe type" oxidation of polychloropyridine carboxylate anions).
In the course of developing the patented process on a pilot plant scale, it has been found that 3,6-D yield and purity are sensitive to the type of graphite the anode is made of. Furthermore, even the most suitable graphite found (Union Carbide grade ATL graphite) undergoes spalling and a drop-off in 3,6-D yield and purity is experienced as the anode ages in use. This is readily corrected by replacing the anode with a fresh one, but that results in higher capital and operating expenses. Accordingly, it is apparent that an otherwise suitable anode material less susceptible to deleterious alterations in prolonged use would be highly desirable.